C++ PATCH to add test to cover case RANGE_FOR_STMT

[Marek Polacek]

Recently it came up that no test in the testsuite triggers the RANGE_FOR_STMT
case in potential_constant_expression_1.  I came up with this valid test that
tickles that codepath.

I can't use ({ }) instead of a lambda because the constexpr machinery doesn't
handle statement expressions; see default: in cxx_eval_constant_expression.

Tested on x86_64-linux, ok for trunk?

2018-10-11  Marek Polacek  <pola...@redhat.com>

        * g++.dg/cpp1z/constexpr-lambda22.C: New test,

diff --git gcc/testsuite/g++.dg/cpp1z/constexpr-lambda22.C 
gcc/testsuite/g++.dg/cpp1z/constexpr-lambda22.C
index e69de29bb2d..8bb473431a5 100644
--- gcc/testsuite/g++.dg/cpp1z/constexpr-lambda22.C
+++ gcc/testsuite/g++.dg/cpp1z/constexpr-lambda22.C
@@ -0,0 +1,20 @@
+// { dg-do compile }
+// { dg-options -std=c++17 }
+
+#define SA(X) static_assert((X),#X)
+
+template<typename>
+constexpr int
+foo ()
+{
+  constexpr int a[] = { 1, 2, 3, 4, 5 };
+  int i = 0;
+  auto j = [&] {
+    for (auto x : a)
+      i++;
+    return i;
+  }();
+  return j;
+}
+
+SA (foo<int>() == 5);